linux-kernel - Re: [x86] 45fc8757d1: BUG:unable_to_handle

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Date:   Fri, 17 Mar 2017 10:49:47 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     kernel test robot <xiaolong.ye@...el.com>
Cc:     Thomas Garnier <thgarnie@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...e.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Jiri Kosina <jikos@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        Len Brown <len.brown@...el.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Michal Hocko <mhocko@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        zijun_hu <zijun_hu@....com>, LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, tipbuild@...or.com,
        LKP <lkp@...org>
Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel

On Fri, Mar 17, 2017 at 4:59 AM, kernel test robot
<xiaolong.ye@...el.com> wrote:
>
> FYI, we noticed the following commit:
>
> commit: 45fc8757d1d2128e342b4e7ef39adedf7752faac ("x86: Make the GDT remapping read-only on 64-bit")
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git x86/mm
>
> in testcase: boot
>
> [    4.347219] BUG: unable to handle kernel paging request at ffffffffff577060
> [    4.360480] Oops: 0003 [#1] SMP
> [    4.373550] RIP: 0023:0xf77e91ed
> [    4.375284] RSP: 002b:00000000ffed034c EFLAGS: 00010246

Heh. That's actually in user space, but the error code (0003) means
"protection fault on a write, not a user access".

So it's almost certainly something that tries to access a segment
descriptor in the GDT, but that segment was marked as "not accessed",
and the CPU was trying to set the accessed bit.

I *thought* we always maked everything accessed when we initialize it,
but something clearly is not.

That's why there's no kernel call trace or anything like that: it is a
system page fault, but it's triggered directly from user mode.

The linear address can be used to look up which entry it is. I assume
the GDT starts at ffffffffff577000, and that this is at offset 0x60
from that. Whatever descriptor that would be..

             Linus